Java SDK: Document char is not supported for XCom parameters

[FrankYang0529]

• Followup from Java SDK: Throw MissingXComException instead of NPE for absent XComs #69257
• Add document about declaring char or Character will get ClassCastException at runtime and suggest to use String instead.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.